Large-Eddy Simulation and Parameterization of Decaying Turbulence in the Evening Transition of the Atmospheric Boundary Layer

This study presents the results of large-eddy simulation (LES) of the evening transition in the atmospheric boundary layer in the case of free convection and in the presence of geostrophic wind. The turbulent kinetic energy (TKE) balance and its components are analyzed. It is shown that within the transition, periods of fast and slow decay can be distinguished. The differences in TKE anisotropy between these two periods are demonstrated. During the fast decay period, the majority of the energy within the vertical component is consumed due to inertial movement of the thermals after the cease of convection. This is followed by the TKE redistribution into large-scale horizontal components, which leads to the formation of quasi-horizontal turbulence, where the TKE dissipation is significantly slower in comparison to the isotropic state. It is shown that one-dimensional boundary-layer model, in which turbulent fluxes are parameterized by means of a two-equation closure, is not able to reproduce evening transition dynamics observed in LES. In particular, the use of the gradient approximation in the one-dimensional model leads to the preservation of the convective distribution of the heat flux along the vertical during the transition period and additional TKE generation in the boundary layer due to the action of buoyancy forces. The use of the phenomenological equation for the dissipation rate leads to decreased TKE decay rate during the fast decay period and increased TKE decay rate during the slow decay period. Possible approaches toward modification of the Reynolds-averaged Navier–Stokes (RANS) closures in order to correctly reproduce transition periods of the atmospheric boundary layer are discussed.